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The absorption spectrum of the lowest triplet state of anthracene is observed in pure crystal at $4.2\circ K$. The following results are thought to be important: 1. The integrated oscillator strength of this state is measured to be $(3\pm 1)\times 10-10$, similar to the triplets in benzene and naphthalene. 2. This transition is polarized most strongly in the direction of the a-axis. The transition moment is perpendicular to the molecular plane. 3. The total exciton splitting in this state is $49cm-1$. However, 30% of the splitting is due to vibronic coupling interaction. 4. Vibrational structures in the triplet state are compared with those in the ground state and the lowest excited singlet state. The Franck-Condon integrals for all vibrations in the triplet are very small. 5. The same transition is observed also in anthracene-$d10$crystal. The additional absorption bands due to anthracene-$d9$ impurities are analyzed according to the exciton theory. The detailed structures in the exciton band of pure crystal are thus inferred.